tools: Use INSTALL_PYTHON_PROG

tools: Use INSTALL_PYTHON_PROG

Whenever python scripts are install, have the shebang be modified to use
whatever PYTHON_PATH is. This is useful for system where python isn't available, or
where the package build tools prevent unversioned shebang.

INSTALL_PYTHON_PROG only looks for "#!/usr/bin/env python".

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Wei Liu <wl@xen.org>